Known head structures of human-shaped robots include one having a head connected to the trunk via four-node link mechanisms comprising left and right pairs (e.g. see Patent Literature 1). Each of these pairs of four-node link mechanisms has a structure that is symmetric between left and right. More specifically, the left and right pairs of four-link mechanisms are configured of lower joints disposed at four corners, forward, backward, leftward and rightward, of a rectangular plate horizontally arranged on the trunk side of the robot, upper joints disposed at four corners, forward, backward, leftward and rightward, of a rectangular frame horizontally arranged on the head side, two links so arranged as to cross each other on the left and right sides (a total of four links, with all the left and right ones counted) and spanning between the left forward upper joints and the left backward lower joints (hereinafter this link may be sometimes referred to as a “drive link”), a link spanning between the right forward upper joints and the right backward lower joints, and a link spanning between the right forward lower joints and the right backward upper joints.
Also, in the left side four-node link mechanism, a drive motor having its output shaft coaxially with the left backward upper joint is arranged, and this drive motor causes the “drive link” to so turn around the output shaft as to form a prescribed angle relative to the left side edge of the rectangular frame. As a result, in the left and right pairs of four-node link mechanisms, the rectangular frame on the head side arranged above the rectangular plate on the trunk side is shifted forward and backward, interlocked with the rotational action of this “drive link”. Further, the left and right pairs of four-node link mechanisms, interlocked with the rotational action of this “drive link”, causes the angle of the rectangular frame on the head side formed relative to the rectangular plate on the trunk side to vary. In other words, by the head structure of this robot, the head is enabled to vary its elevation/depression angle, namely to make a nodding action, being interlocked with the forward and backward shifting of the head.